Dispensers for liquid material are well known in the art. Such dispensers may be of the piston type whereby a piston is advanced along a cylinder reservoir to dispense material. Syringe type reservoirs with needle-shaped dispenser tips frequently employ a piston to discharge material. In other types of dispensers a deformable tube may be employed with a pinch valve to control the dispensation of material. Such dispenser may be as described in a copending patent application entitled "Deformable Tube Material Dispenser" by Paul S. Citrin, the same inventor as of this invention, and filed on Apr. 14, 1975 with Ser. No. 568,123, now U.S. Pat. No. 3,982,724.
In the dispensation of highly viscous materials such as solder pastes, adhesives, greases and the like, a need exists to be able to discharge small amounts in an accurate and reliably repetetive manner. For example, a common problem encountered with piston driven dispensers involves the reliable dispensation of small quantities whose separation from the main stream in the dispenser is difficult to control. When dispenser parts are exposed the abrasiveness of some of the materials quickly wears out these mechanical components. The dispenser's resistance to material flow frequently makes it difficult to control the dispensation of precise quantities and may accelerate viscous material compacting problems when forced through valves and other tortuous paths to a discharge port.
Frequently material characteristics inhibit crisp cut-off of the dispense cycle and make clean-up procedures extremely difficult, particularly when the viscous material has a limited pot life and thus must be dispensed expenditiously. Other materials exhibit a tackiness or stringing tendency, thus making a crisp cut-off difficult to achieve. Such problems in the dispensation of viscous materials effectively inhibit their uses in many areas and deny to potential users the benefits these materials offer.